1. Field of the Invention
The present invention relates generally to signal reception, and more particularly to selection of received signals.
2. Description of the Related Art
Cochannel signal interference occurs when two or more signals are received at the same time over the same frequency range. For example, cochannel signal interference may be encountered by a receiver that is receiving two or more signals transmitted at the same frequency and at the same time by two or more separate transmitters. In such a case, data (e.g., voice data, text data, etc.) contained in one or more of the interfering cochannel signals cannot be accessed or processed further without first separating the given signal from the other signals to allow demodulation or other further signal processing. This situation occurs frequently with airborne receivers and also occurs with ground system receivers as well. Even signals with carefully planned frequency re-use (such as radio stations), often result in co-channel interference for airborne receivers due to the much longer line-of-sight.
In the past, beamforming and interference cancellation techniques have been employed for purposes of cochannel signal separation. In one example beamforming technique, multiple sensors are employed to separate and copy multiple candidate signals of the same class that are transmitting on the same frequency range or band. These candidate signals include a signal of interest and cochannel interferer signals. In the case of a signal of interest that is a sonobuoy signal, these captured candidate signals are then output as audio on multiple audio ports for a human operator to monitor, and for further acoustic processing of the signal of interest that is selected by the human operator. Besides requiring a human operator to monitor and select the signal of interest, in a non-stationary environment, such as an airborne beamformer platform, the extracted signals can sometimes swap with each other (referred to as “port-swapping”) producing the undesirable effect of an operator losing the selected signal being monitored. Port-swapping can also occur when multiple signals become co-linear with respect to the collection sensors. This phenomenon causes disturbances in acoustic processing of the sonobuoy signal, and may require many seconds of additional processing to recover from the disturbance.
Signal transmitting devices have been employed to transmit digitally coded identification information within or in combination with transmitted signals. Although transmitting such identification information enables signals to be identified and assigned to correct ports, it increases the complexity of transmitting devices, requires transmitting devices to be compatible with the communications network by transmitting digitally coded information in the correct format, and increases system complexity by requiring the receiving system to continuously parse and decode information from each signal.